My suggestion would be to look at some of the kensington automotive laptop adapters, they have a few to pick from, and they're a one-adapter-fits-all type of solution. There are different wattages available, it might be good to know what laptop you're trying to power.

My google-fu is strong today. This adapter does it all, wall power, automotive power, and airplane power. A DC to DC like that would be good. I'm not saying kensington is the end-all, that's just the only manufacturer I could think of off the top of my head that made a generic one.

I got this one not too long ago for under $25 bucks and it seems to work as you would expect it to. It stays pretty cool, since I had been shopping for one with a much higher wattage than my puter even needed: